In the case of web-fed printing machines, the ink feed is spread over the width of the printing form in varying amounts in columns. The thickness of the ink layer in the individual columns is determined by the gap between an inking roller and its corresponding doctor blade, which is in the form either of a flexible individual element or alternatively of several individual slides arranged side by side and moving independently of one another. The flexible doctor blade is, or the slides are, adjustable to the desired position in relation to the inking roller by electric servomotors.
The position of the blade or slides relative to the inking roller is electrically measured by sensors and indicated on a remote-control operator's console.
In the case of large newpaper printing machines with many printing units, up to 1000 individual gap sensors have to be precisely adjusted before putting the machine into operation. As practice has shown, this is very time consuming since the sensors have to be mechanically set precisely to one-hundredth of a millimetre and their related signal amplifiers so coordinated that all column gaps can be measured with the same zero point, same scale and same exactness.
Since not only the flexible doctor blades but also the alternative slides as well as the inking rollers wear in the course of time, readjustment is frequently necessary, especially when the wear necessitates complete replacement. As a rule this entails considerable work with correspondingly high costs.